Pin dependencies from the dotnet/efcore repository

[wtgodbe]

In the spirit of pulling EFCore out of our release graph, we should stop having those dependencies be updated automatically by DARC, and instead treat them like 3rd party dependencies. We can do this one of two ways, both of which require deleting the Darc subscriptions:

1. Pin the dependencies in version.details.xml, update both version files when we want to take an EFCore update
2. Remove the dependencies from version.details.xml entirely, update Versions.props when we want to take an EFCore update

I prefer option 2, but am open to persuasion. I think removing the dependencies from version.details.xml creates a better representation of what our "graph" looks like, since that's the file that exactly represents what's updated through DARC. It also gets rid of some weird potential bugs related to CoherentParentDependencies - e.g. if we ever declare Microsoft.NetCore.App as the parent for a CPD, darc could choose the EFCore dependency on Microsoft.Netcore.App as the "parent", and do the wrong thing. This happened in #18247.

CC @bricelam @ajcvickers @dotnet/aspnet-build

[dougbu]

Why make the EF dependency flow into aspnetcore manual at this time? As far as I know, the eventual story for 3rd party dependencies will not be manual. This would add extra work and not simplify much, if anything.